Heat pump water heaters are gaining broader acceptance as a more economic and ecologically-friendly alternative to electric water heaters. Heat pump water heaters include a sealed system for heating water to a set temperature. The sealed system generally includes a condenser configured in a heat exchange relationship with a water storage tank within the water heater appliance. For example, the condenser may be wrapped around the water storage tank in a series of coils. During operation of the sealed system, a refrigerant exits an evaporator as a superheated vapor and/or high quality vapor mixture. Upon exiting the evaporator, the refrigerant enters a compressor where the pressure and temperature increase and the refrigerant becomes a superheated vapor. The superheated vapor from the compressor enters the condenser, wherein the superheated vapor transfers energy to the water within the water storage tank and returns to a saturated liquid and/or high quality liquid vapor mixture.
During operation of the sealed system, water vapor can condense or desublimate on the evaporator and form a frost buildup over time. The frost buildup can negatively affect performance of the sealed system. To remove the frost buildup from the evaporator, heat pump water heater appliances are generally configured for performing a defrost cycle periodically. As an example, certain heat pump water heater appliances include heating elements mounted to the evaporator that are activated during the defrost cycle to melt the frost buildup. Operating heating elements during the defrost cycle can be energy intensive and negatively affect an efficiency of such heat pump water heater appliances. As another example, certain heat pump water heater appliances include a reversing valve for directing heated refrigerant from the compressor to the evaporator in order to melt the frost buildup. Melting the frost buildup in such a manner requires operating the compressor and can also require an additional, expensive valve.
Accordingly, a method for defrosting an evaporator of a heat pump water heater appliance efficiently and/or economically would be useful. In particular, a method for defrosting an evaporator of a heat pump water heater appliance without requiring heating elements on evaporator or operating a compressor of the water heater appliance would be useful.